This invention relates to ultrasonic flowmeters and, more particularly, to a method and apparatus for the ultrasonic measurement of the flow velocity of fluent media.
Various ultrasonic flow measuring devices are known in which signal frequencies are related to the downstream speed of sound f.sub.1 and the upstream speed of sound f.sub.2. Typically, a voltage controlled oscillator is used to provide a reference signal after a predetermined number of oscillations. A comparator compares the actual arrival time of the transmitted ultrasonic signal to that of the reference signal. If this actual arrival time is earlier than the time of occurrence of the reference signal, the frequency of the oscillator is increased to adjust the timing of the reference signal to occur at the actual arrival time of the ultrasonic signal. If, on the other hand, the actual arrival time of the ultrasonic signal is later than the reference signal, the frequency of the oscillator is reduced. The frequencies f.sub.1 and f.sub.2 may be processed to determine flow velocity (f.sub.1 -f.sub.2)/2) or sonic velocity (f.sub.1 +f.sub.2)/2.
In another flowmeter, such as that described in U.S. Pat. No. Re. 28,929, a signal level generator is added to the timing circuitry. Signals are applied to the input of the signal level generator to vary the signal level according to whether the respective upstream and downstream arrival signals are early or late relative to a reference signal. Even here, a voltage controlled oscillator, whose frequency is selected as a measurement of the sonic velocity, is used to control the transmission of the signals and also the reference signal. An improvement over this sytem is described in U.S. Pat. No. 3,981,191.
All of the described ultrasonic flow measuring systems, as well as an older flow measuring system based on the so-called "sing-around" principle, have one feature in common. This feature is that there is either a signal level generator or oscillator functioning as a timing generator for providing the reference criteria from which sound speed and flow speed of the fluid media are obtained.
A problem often arises in using these ultrasonic flowmeters in that sometimes the desired degree of sensitivity is not always as high as one might wish to attain. This is particularly true in measuring flow in larger diameter pipes. Because of the longer transmission times of the ultrasonic signals in large pipes, small changes in flow rate constitute a smaller part of the total transmission time. Sensitivity is hence decreased.
Accordingly, it is an object of this invention to provide a method for increasing the sensitivity of ultrasonic flowmeters.
Another object of this invention is to provide an improved apparatus for increasing the sensitivity of ultrasonic flowmeters.